1. Field of Invention
This invention relates to improvements in power amplifier methods and apparatuses that are suitable for use in both audio devices and devices for dynamic information storage or retrieval. More particularly, this invention in one aspect relates to improvements in methods and circuitry for controlling the position of the data transducer, or head, used in mass data storage devices, hard disk drive devices, or the like.
2. Relevant Background
Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected to further increase in the future. Mass data storage devices may also include optical disks in which the optical properties of a spinning disk are locally varied to provide a reflectivity gradient that can be detected by a laser transducer head, or the like. Optical disks may be used, for example, to contain data, music, or other information.
The data transducer or head used in mass data storage devices is selectively positioned to desired locations of the disk by a voice coil motor (VCM). The circuitry that provides drive signals to the VCM typically has a pair of high and low side driver circuits. In operation, each set of high and low side drivers is connected on respective opposite sides of the VCM, and operate in a manner in which a high side driver from one set is activated together with a low side driver of the other set to drive a current through the VCM in one direction to move the head in a respective first direction. When the respective high and low side drivers of the opposite sides are activated, a current is driven in the opposite direction through the VCM to move the head in the opposite direction.
Very low distortion is required in power amplifiers used specifically in hard disk drive servo systems as well as audio systems. When a full NMOS H-bridge operates a voice coil at or around the null, or xe2x80x9cdead bandxe2x80x9d or xe2x80x9cdead zonexe2x80x9d area, significant distortion can result. This distortion normally occurs because the H-bridge driver FETs turn off in this zone of operation. In the past, a Class-AB mode of operation has been imposed onto the H-bridge transistors by brute force techniques. For example, one way that has been used is to switchably apply a gate voltage onto the H-bridge transistors during crossover from a high side driver to a low side driver, or vice versa. This technique is generally uncontrollable, and often does not result in the desired harmonic distortion or other desired switchover characteristics.
What is needed therefore, is a means for controlling the operation of the power FETs of an audio amplifier of the type used in mass data storage devices, or the like, so that at switchover between the high and low side drivers, a Class-AB mode of operation can be controllably maintained.
In light of the above, therefore, it is an object of the invention to provide a Class-AB driver circuit for a VCM of a mass data storage device in which better control and predictability of the Class-AB quiescent current can be achieved.
It is another object of the invention to provide a Class-AB driver circuit of the type described in which predictable power consumption and crossover linearity of the VCM current can be achieved.
It is still another object of the invention to provide a Class-AB driver circuit of the type described that has lower harmonic switchover distortion.
One advantage of the invention is that Class-AB operation of a power amplifier can be maintained and controlled during switchover, without the occurrence of discontinuities thereat.
It is another advantage of the invention that the total harmonic distortion of the circuit can be greatly reduced due to the elimination of any discontinuities at the Class-AB switchover point.
It is another advantage of the invention that the power dissipation of the circuit during switchover is greatly reduced from previous circuits.
Another advantage of the invention is that it can be practiced with either external or integrated H-bridge transistors, without a strict requirement for parameter matching therebetween.
Yet another advantage of the invention is that it enables Class-AB operation to be maintained as the high and low side H-bridge transistors are switched through the xe2x80x9cdead zonexe2x80x9d, which results in less distortion and fewer harmonics.
It is yet another advantage of the invention to provide a method and circuit in which a wider Class-AB range of operation can be achieved, which results in lower distortion at higher operating frequencies.
It is still yet another advantage of the invention to provide a method and circuit in which a more precise quiescent current due to separate control of the quiescent current of each power FET, which results in higher power efficiency, as well as lower distortion.
These and other objects, features, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.
According to a broad aspect of the invention, a circuit is provided for applying drive voltages to a voice coil motor (VCM), such as that of a mass data storage device, or the like. The circuit has two driver sets. Each driver set has a high side driver and a low side driver. Each driver set is connected to respective opposite sides of the VCM. Each driver set has first and second SENSEFET devices respectively in the high side driver and the low side driver. Each SENSEFET device has a first portion for selectively connecting a side of the VCM to a VCM driving potential and a second portion for sensing a current in the first portion. The first and second portions may be, for example, a power FET portion and a sense FET portion. A circuit is provided for sensing a sense current in the second portion of the SENSEFET device of the low side driver, and a circuit is provided for increasing the bias on the gate of the SENSEFET device in the low side driver when the sense current falls below a predetermined level. In addition, a circuit is provided for driving a predetermined current in the SENSEFET device of the high side driver when the sense current falls below the predetermined level. Thus a current of at least about the predetermined level always flows in the SENSEFET devices.
According to another broad aspect of the invention, a circuit is presented for maintaining a Class-AB mode of operation of an H-bridge voice coil motor (VCM) driver. The circuit has four SENSEFET devices connected in an H-bridge configuration with the VCM. Four gate biasing circuits are connected to bias respective gates of the SENSEFET devices to selectively turn the SENSEFET devices on and off in response to positioning signals for the VCM. Sensing circuitry is associated with respective the SENSEFET devices to operate the gate biasing circuits to maintain gate biases on the SENSEFET devices when currents in the SENSEFET devices fall below a predetermined level to maintain the circuit in Class-AB mode of operation.
According to yet another broad aspect of the invention, a circuit is provided for applying drive voltages to a voice coil motor (VCM), such as that of a mass data storage device, or the like. The circuit has two driver sets, each including a high and a low side driver. Each driver set is connected to respective opposite sides of the VCM. Each driver set has first and second SENSEFETs respectively in the high and low side drivers. Each SENSEFET has a power FET portion for selectively connecting a side of the VCM to a driving voltage potential and a sense FET portion for sensing a sense current in the power FET portion. Means are provided for developing a sense voltage proportional to the sense current sensed by the sense FET portion of the SENSEFET of the low side driver. Means are also provided for increasing a gate bias on gates of the SENSEFET of the low side driver when the sense current falls below a predetermined level. Means are additionally provided for driving a predetermined current in the power FET portion of the SENSEFET of the high side driver when the sense current falls below the predetermined level. Thus, at least about the predetermined current flows in the SENSEFETs at least during a switchover from a high side driver to a low side driver in one of the driver sets.
According to yet another broad aspect of the invention, a method is presented for providing drive voltages to a voice coil motor (VCM). The method includes configuring four SENSEFET devices in an H-bridge configuration with the VCM to provide two driver sets with each driver set including a high side driver and a low side driver. The method also includes biasing the SENSEFET devices to conduct a minimum quiescent current during a switchover between a high side driver and a low side driver in one of the driver sets.
According to yet another broad aspect of the invention, a mass data storage device is presented. The mass data storage device includes a circuit for providing drive voltages to a voice coil motor (VCM) and has two driver sets. Each driver set has a high and a low side driver for connection to respective opposite sides of the VCM. Each of the driver set includes first and second SENSEFET devices respectively in the high and low side drivers. Each SENSEFET device has a first portion for selectively connecting a side of the VCM to a VCM driving potential and a second portion for sensing a current in the first portion. A circuit is provided for developing a sense voltage proportional to a sense current sensed by the second portion of the SENSEFET device of the low side driver, and a circuit is provided for increasing a gate bias on gates of the SENSEFET device of the low side driver when the sense current falls below a predetermined level. A circuit is also provided for driving a predetermined current in the SENSEFET device of the high side driver when the sense current falls below the predetermined level. Thus, a current of at least about the predetermined level always flows in the SENSEFET devices.